Virtually every computer, appliance, or portable device makes use of mainly three different memory technologies to store and retrieve information: (1) a magnetic hard disk (HD), (2) dynamic random access memory (DRAM), and/or (3) FLASH memory technologies. The combined use of as many as three different technologies is generally due to the need to overcome various limitations exhibited by each individual technology.
For example, magnetic HDs provide the largest possible data density, yet magnetic HDs make use of sophisticated mechanics to enable such high data density. This sophisticated level of mechanics results in at least three unfortunate consequences. In particular, a magnetic HD is slow, energy consuming, and quite delicate, the last two limitations being largely unacceptable in portable devices. Semiconducting FLASH memories, widely used in pen drives and memory cards, are also non-volatile memory, yet do not suffer from the same fragility as do magnetic HDs. As a result, FLASH memory technologies, even though they cannot achieve a similar data density, are considered to be the best candidates to replace magnetic HD drives.
As a matter of fact, solid state HDs based on FLASH technology are already available in the marketplace today. However, after only a few years from their initial commercialization, such solid state HDs already appear to have reached a data density limit, despite the fact that the memory capacities remain about one order of magnitude smaller than that of a standard magnetic HD. In principle, this data density limit of FLASH memory technologies is the same as that limiting the performance of the very fast DRAMs. In particular, in both cases, information is retained by storing charge and the charge always leaks, either through the tunnel barrier, when it is made as thin as possible to increase velocity, like in DRAMs, or through the barrier sidewalls, when the bit cells are made as small as possible to increase density, like in current solid state HDs.
This consideration appears to suggest that memories of the future might still make use of magnetic materials, given such materials do not suffer from charge dissipation/leaks. For example, by using such materials, one does not have to rely on the charge of the electrons. Rather, one can rely on the magnetic moment. Such is another aspect of the revolutionary concept behind spintronics, first introduced by Albert Fert (Baibich et al., Physical Review Letters, volume 61, pages 2472-2475) and Peter Grunberg (Binach et al. Physical Review B, volume 39, pages 4828-4830), which garnered a Nobel Prize award in 2007.
In view of these and other recent breakthroughs, a number of devices have been proposed which make use of magnetic bit cells and yet do not have fragile mechanical parts. Such magnetic bit cells are usually referred to as magnetic random access memories (MRAMs), since they combine the non-volatility of the magnetic memories with the functionalities of the random access memories. In these devices, while reading is easily accomplished by using any of the magnetoresistive or Hall effects, information is written by either reversing the entire orientation of the magnetization of patterned magnetic structures (see, for instance, Schuster-Woldan et al., US 2001/0035545 (A1)) or by displacing domain walls between stable positions, usually represented by geometrical constrictions (see, for instance, Bland et al. U.S. Pat. No. 7,102,477 (B2) and Wunderlich U.S. Pat. No. 6,727,537 (B2)). Another possibility recently proposed is to switch either the core or the chirality of a magnetic vortex in a magnetic dot (see, for instance, Min et al. US 2006/0023492 (A1)).
However, in all these cases, density is limited by electromagnetic cross-talk effects, when the writing process relies on the local magnetic field produced by the current flowing on the address lines (Nozaki et al. Journal of Applied Physics, volume 93, pages 7295-7297). As an alternative, writing can rely on spin transfer torque effect (Slonczewski, Journal of Magnetism and Magnetic Materials, volume 159, pages L1-L7, Huai et al. U.S. Pat. No. 7,106,624 (B2)) but, so far, this requires large current densities, with a consequent limitation of the bit density due to Joule heating.
Accordingly, a challenge remains in the field of magnetic bit cells to find scalable magnetic structures that can be switched between two stable magnetic configurations while using small write currents.